1. Field of the Invention
The present invention relates to a heating element, at least employing a substrate, an electrically insulating layer, and a resistive layer. Moreover, the present invention relates to an electrical domestic appliance including such a heating element, as well as to a method of manufacturing said heating element.
2. Description of the Related Art
A known heating element known from U.S. Pat. No. 5,822,675, which discloses a heating element that employs a substrate having on its surface a first layer of a silicon based electrically insulating material. Preferably, the layer is also thermally conductive to transfer a high amount of heat from the electrically resistive layer. To achieve both electrical insulation and thermal conductivity, the first layer preferably includes a filler, such as, for example alumina, silicon carbide or zirconium diboride, in addition to the silicone resin. On a surface of the first layer is a second layer including a silicon based electrically resistive material. Attached to the second layer are at least two separate areas of silicon based electrically conductive material. Each of these separate areas are suitable for connection to a power supply.
A disadvantage of the above heating element is that silicon used in the electrically insulating layer leaks as it takes up water, and therefore exceeds the maximum allowable leakage current of 0.75 mA as specified in IEC 335-1.
It is an aim of the present invention to provide for a heating element that has a relatively high breakdown voltage. In particular, the present invention aims to provide for a heating element, the substrate of which comprises aluminum or anodized aluminum. It will be clear that the term aluminum comprises both aluminum and alloys of aluminum. Furthermore, the present invention aims to provide for an electrical domestic appliance including such a heating element, as well as to a method of manufacturing the heating element.
These and other objects of the invention are achieved by a heating element which is characterized in that the electrically insulating layer comprises a layer which is obtained by a sol-gel process.
By applying an electrically insulating layer comprising a so-called sol-gel layer several advantages are achieved. First of all, the sol-gel layer shows excellent electrically insulating properties. The carbon content of sol-gel materials is sufficiently low to prevent the formation of a carbonized conductive track in case of failure of the heating, thereby providing a safe heating element. Also, sol-gel materials have a high thermal conductivity which is in the order of magnitude of 0.5-2 W/m/xc2x0 K. Furthermore, sol-gel material can be processed at temperatures below 400xc2x0 C., which makes this material suitable to be applied directly to aluminum substrates.
Although the sol-gel electrically insulating layer is especially suitable for application on aluminum substrates, other substrates which are conventionally used for heating elements and which are compatible with the final utility may also be used. The substrates may include, for example, stainless steel, enameled steel or copper. The substrate may be in the form of a flat plate, a tube or any other configuration that is compatible with the final utility.
In particular, the sol-gel process at least comprises the step of mixing an organosilane compound with water.
The organosilane compound advantageously contains at least two hydrolytically condensable substituents.
Preferably, the sol-gel process comprises the step of mixing an organosilane compound and silica particles, in particular colloidal silica particles.
Although several organosilanes can be used, for high temperature applications heat resistant silanes are preferred. Preferred examples of such silanes are methyltrimethoxysilane (MTMS) and tetraethoxysilane (TEOS). An increase of the layer thickness of such layers can be obtained by the addition of fillers, such as colloidal silica.
In some cases, it is necessary to have a certain minimum thickness of the electrically insulating layer, for example in view of the capacitive functioning of the layer. In order to obtain such thickness, the electrically insulating layer may also comprise a resin.
This can be advantageous, in particular when the sol-gel material cannot be applied in the desired thickness.
In order to obtain a screen printable electrically insulating layer, the layer advantageously comprises a resin with an insulating filler.
The insulating fillers may comprise titanium oxide, silicon oxide, aluminum oxide, mica or iron oxide.
Preferably, the resin comprises polyimide.
Advantageously said polyimide may be filled with an insulating filler.
Polyimide is a material with a high temperature resistance and with good electrically insulating properties. The material shows no leakage up till 350xc2x0 C. The insulating filler material may comprise any insulating filler as mentioned above.
However, in case of extreme load or at the end of life, the heating element may fail. In the process of failure, the polyimide insulating layer may carbonize, thereby forming an electrically conducting track. It will be clear that this can result in dangerous situations.
In order to avoid such dangerous situation, the electrically insulating layer of the heating element preferably comprises at least a first layer which is obtained by a sol-gel process, as well as a second layer which comprises a thermoplastic resin, preferably polyimide.
In case of a breakdown voltage of the polyimide layer, the sol-gel layer will remain insulating and will act as a security of the heating element.
The present invention further relates to an electrical domestic appliance comprising at least a heating element in accordance with the present invention.
The heating elements according to the present invention are especially suitable for use in applications where high temperatures are used. These applications comprise, for example, (steam) irons, kettles, coffee makers, deep frying pans, grills, space heaters, waffle irons, toasters, ovens, water flow heaters, and the like.
Finally, the present invention relates to a method of manufacturing a heating element, at least comprising the steps of: providing a substrate; applying an electrically insulating layer on said substrate; and applying a resistive layer on top of the electrically insulating layer.
The method is characterized in that the electrically insulating layer comprises a layer which is obtained by a sol-gel process.